Black dwarf

A Black dwarf is the remains of a sun-sized star which has evolved to a white dwarf and subsequently cooled down such that it no longer gives out radiation. None exist in our universe, as the time taken for a white dwarf to cool to such a degree is longer than the lifespan of the universe to date.

Even at the epoch when black dwarfs exist, they will be extremely difficult to detect, since they will emit little radiation above thermal radiation at a temperature not much above that of the cosmic microwave background radiation. The only way to detect them will be through their gravitational influence (which, being low mass stars, will not be much).